Project Summary There is a well-recognized need to develop sensitive and selective biosensors for early detection and prognosis of disease and to identify more effective drugs to treat these diseases. Drug development is a billion-dollar process that can take over a decade. Development of high throughput and low cost sensors to screen drug candidate libraries could significantly enhance the efficiency of drug development. Of particular interest are sensors that can detect direct binding of small molecule ligands (<1kD) to receptor targets without the need for labels such as fluorescent tags, as tagging is not always possible for the ligand of interest, adds additional cost, and may alter the binding behavior. We have recently demonstrated that microtoroid optical resonators can detect low concentrations of unlabeled proteins, down to the single macromolecule level; however, in order to successfully translate these devices out of the laboratory and into applications such as drug screening and detection in biological samples, these transducers need to be adapted to sensitive and selective detection of small molecules at low concentrations, ideally in chemically complex biofluids. To address this challenge, we propose to create a new class of optical biosensors based on a microtoroid transducer coated with an artificial lipid membrane functionalized with G-protein coupled receptors (GPCRs). This novel approach will enable highly sensitive and selective detection of ligands that target GPCRs, such hormones, neurotransmitters, and their analogs, spawning potentially disruptive implementations in drug library screening, cell signaling studies, and clinical assays. Ligand binding to a GPCR causes activation of the receptor, inducing a conformational change. Previous work has demonstrated that the conformational change and the associated local perturbation to the surrounding lipid bilayer produces a change in the optical thickness of the proteo-lipid membrane that can be detected directly, in a label-free manner, using plasmon-waveguide resonance (PWR) spectroscopy. The microtoroid optical resonator platform proposed here is much more sensitive than PWR spectroscopy. In this proposal, we will specifically target the kappa-opioid GPCR which regulates reward and mood. Because reward and mood affect addiction, depression, and pain, the kappa-opioid receptor is an important drug target. The World Health Organization (WHO) estimates that 350 million people suffer from depression alone. Better drug treatment options are needed, as antidepressants have unpleasant side effects, including in some cases tremors, drowsiness, and increased risk of suicide. The sensing concept described herein will address this need by constructing and demonstrating the feasibility of a kappa-opioid GPCR/artificial membrane/microtoroid sensor, which will open the way for future development of numerous other types of GPCR-based small molecule sensors.